1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device, a plasma oxidation treatment method, and the like.
2. Description of the Related Art
As one type of field-effect transistor, a thin film transistor whose channel region is formed using a silicon film which is formed over a substrate having an insulating surface is known. Techniques in which amorphous silicon, microcrystalline silicon, or polycrystalline silicon is used for forming the silicon film which is used for the channel region of the thin film transistor have been disclosed (see Patent Documents 1 to 5). A typical application of the thin film transistor is a liquid crystal television device, in which the thin film transistor is practically used as a switching transistor in each pixel in a display screen.
FIG. 13 is a cross-sectional view of an inverted staggered bottom-gate thin film transistor. This thin film transistor includes a gate electrode 4 having a stacked structure and formed over a glass substrate 10; a gate insulating film 2 formed of a SiNX film that is formed to cover the gate electrode 4; a stacked film in which a microcrystalline silicon film 1, a silicon film 3, and a pair of impurity silicon films 5a and 5b are stacked over the gate insulating film 2; wirings 6a and 6b having a stacked structure over the impurity silicon films 5a and 5b and functioning as a source and drain electrodes; an insulating film 7 formed to cover the wirings 6a and 6b; and a gate electrode (hereinafter referred to as a back gate electrode) 8 formed over the insulating film 7.
In the case of forming an insulating film containing nitrogen such as a SiNX film as the gate insulating film 2, nitrogen in the gate insulating film 2 inhibits crystallization at the time of forming the microcrystalline silicon film 1 on an upper portion 2a of the gate insulating film 2, whereby an amorphous silicon region (so-called incubation layer) is formed on the gate insulating film 2. This silicon region may cause degradation of characteristics of the thin film transistor.
At the time right after a portion 3a, which is located on the back channel side in the silicon film 3, is exposed by dry etching the silicon film 3 over the microcrystalline silicon film 1, a silicon region in the surface of the portion 3a in which the back channel is formed is damaged by the etching; this may result in a degradation of characteristics of the thin film transistor. Note that the back channel means a region facing the back gate electrode and positioned in the microcrystalline silicon film 1 or the silicon film 3.